It looks like it is in the iteration plan for June, 2019.
See Iteration plan for June, 2019: release in July, 2019 and issue: merging single child directories.
Update: making it into v1.41 to be released in December, 2019. See https://github.com/microsoft/vscode-docs/blob/vnext/release-notes/v1_41.md#compact-folders-in-explorer
Compact folders in Explorer
In the File Explorer, we now render single child folders in a compact
form. In such a form, single child folders will be compressed in a
combined tree element. Useful for Java package structures, for
example.Setting
explorer.compactFolders
controls this behavior. By default,
this setting is turned on.
So "explorer.compactFolders": false,
will disable the feature.